Hard disk and virus problems

I am disturbed with computer problems these days. Last week, hard disk crashed and this week I am having virus problems. I am trying to fix both the problems but could not. I heard that hard disk crashes due to electric failure or virus or due to other problems. Before it crashed, I worked as usual and shutdown the computer normal. Next day, when I turn on the computer, it didn't start as usual and took a long time to boot. I push the restart bottom and tried to boot again but it started saying problem like 'disk error' or something. Then I could not boot the computer at all. I unplugged the hard disk and checked with other computer. The computer starts slow but I could not see the recently connected harddisk. I checked the hard disk with system bios while turning on the computer and it shows my both harddisk and their sizes. I don't understand the problem why system bios shows it and not the system. I tried with some recovery software but without seeing harddisk in the system, they could not do anything. Lastly, I asked hardware specialist but he could not do anything too. The disk is gone as well as my data!!!

Because of harddisk failure, I was advised to have updated antivirus in the system. I am trying to scan everything (harddisk, pen drives, portable drives etc) before using in my computer. While scanning with Avira Antivir, it prompt with "infectd with ' W32/Sality' virus". It also says that the virus is removed. But when I tried to scan the same directory again, same virus on same files. No matter how many times I scan, I have the same problem. Is there any way out for it?
In another, memory drive, I got "TR/Crack.A", "TR/Agent.20250" and "TR/Agent.76288.C" infection. While checking about these virus/Trojons, I could not get details.

If you have any solutions to above problems (harddisk and virus), please share with me.